Interconnect (RC) delay is known to be a major limiting factor in the drive to improve the speed and performance of integrated circuits (IC). For performance and cost reasons, it is desirable to have adjacent conductors as close as possible to one another. Since interconnect delay depends on both the dielectric constant of the insulating material that separates the conductors and the thickness of this insulating material, the interconnect delay can be reduced by using low dielectric constant (low-k) materials. Low-k dielectric materials refer to those insulating materials that have a dielectric constant (k) lower than that of silicon dioxide (SiO2) (k=3.9) and generally having k≦3.
In recent years, low-k materials that have been developed to replace relatively high dielectric constant insulating materials, such as SiO2, have been utilized in the fabrication of integrated circuits. Unless otherwise noted, all k-values mentioned in the present application are measured relative to a vacuum. In particular, low-k films are being utilized for inter-level and intra-level dielectric layers between metal layers of semiconductor devices. Additionally, in order to further reduce the dielectric constant of insulating materials, low-k dielectrics can be formed with pores, commonly referred to as porous low-k dielectric films. Porous low-k films which provide k<2.6 are generally referred to as ultra low-k (ULK) films.
Low-k dielectric films, including ULK films, can be deposited by spin-on dielectric (SOD) methods similar to the application of photoresist, by chemical vapor deposition (CVD) methods, or by plasma enhanced CVD (PECVD) methods. Thus, the use of low-k materials is readily adaptable to existing semiconductor manufacturing processes. Examples of typical low-k dielectric materials include fluorine-doped silicon dioxide (also referred to as fluorinated silicate glass, or FSG), organosilicate glass (OSG), thermoplastic organic polymers, aerogel, xerogel, and other conventional low-k insulator materials.
However, these low-k dielectric films also present many fabrication challenges. First, low-k dielectric films tend to be less robust than more traditional dielectric layers and can be damaged during certain wafer processing steps, such as by plasma etch and plasma ash processes generally used in patterning the dielectric layer, as well as from barrier/seed deposition processes and chemical-mechanical polishing (CMP). Further, some low-k films tend to be highly reactive when damaged, particularly after patterning, thereby allowing the low-k material to absorb water and/or react with other vapors and/or process contaminants that can alter the electrical properties of the dielectric layer. As a result, low-k dielectric films, originally having a low k-value, as applied, can suffer damage leading to an increase in its k-value and other detrimental effects.
For example, an important class of low-k dielectric materials is referred to as substitution-group depleted silicon oxide. By way of definition, these low-k materials have the chemical formula R1R2SiOx, where R1 and R2 refer to hydrogen, oxygen, a methyl group (—CH3), an ethyl group (—CH2—CH3), a phenyl, or a dangling bond. Such low-k dielectric films, such as OSG films, provide a lower dielectric constant than stoichiometric SiO2. However, exposing such low-k dielectric films to plasma etch processes can damage the surface of the exposed low-k dielectric films. Similarly, plasma ash processes for removing masking materials, such as photoresist, that define the trench and via locations, can also damage the exposed surfaces of the low-k dielectric. For example, the plasma ash damage in these low-k dielectric films, such as OSG films, can be caused by a chemical reaction between constituents of the material and the excited species in the plasma. Specifically, the low-k dielectric film can be damaged by the conversion of silicon-hydrocarbon bonds in the material to silicon-hydroxide bonds when the material is exposed to oxidizing or reducing plasmas. Examples of these undesirable reactions are shown in FIGS. 1B through 1D.
FIG. 1A illustrates, in cross-section, a portion of a partially formed integrated circuit. Underlying structure 3 refers, in a general sense, to underlying structures and layers over which a subsequent metal layer will be formed. As such, underlying structure 3 may include the underlying semiconductor substrate and any epitaxial layers, wells, and doped regions formed at a surface of the substrate, overlying dielectric layers, conductive levels including polysilicon or metal gate layers and levels, and previous metal levels, including refractory metals, copper, or aluminum metallization. A low-k dielectric film 2, such as an OSG film, is applied over underlying structure 3. In an OSG dielectric film, silicon atoms are bound to three oxygen atoms and to one methyl group (CH3), corresponding to the organic content present in an OSG film. As evident from FIG. 1A, the low-k dielectric film 2 has been etched, using a masking layer 4 defining the features to be etched into the low-k dielectric film 2. Such masking layers are generally photoresist layers, hardmask layers, or any combination thereof, At the exposed locations, low-k dielectric film 2 is etched through to underlying structure 3. Alternatively, for example in the formation of a conductor trench, dielectric film 2 may only be partially etched through at this location.
FIGS. 1B and 1C illustrate, respectively, the reaction of low-k dielectric films, such as OSG films, resulting from a subsequent oxidizing and reducing plasmas, used to remove the remaining portions of photoresist 4 from the surface of dielectric film 2. Oxidizing plasmas, using plasma activated oxygen or CO2, and reducing plasmas, using plasma activated hydrogen, are commonly used in the art for photoresist removal. In either plasma, the bonds between the silicon atoms and the methyl groups in these surface molecules are typically lost or broken. In an oxidizing plasma, hydroxyl groups, as shown in FIG. 1B, may replace the lost methyl groups. Alternatively, in a reducing plasma, as in FIG. 1C, the bonds can be either left dangling (i.e., associated with an unpaired electron spatially localized at the site of the removed methyl group) or a hydrogen atom can attach to the silicon atom in the place of the removed methyl group. During subsequent processing, water molecules can react with the dangling bonds, as shown in FIG. 1D and attach hydroxyl groups either to the dangling bonds at the exposed surfaces of the low-k dielectric film 2, or replacing the hydrogen molecules that bonded to the silicon atom after the hydrogen plasma exposure.
In either case, the plasma process can degrade the integrity of low-k dielectric films 2, such as OSG films. One form of degradation which can increase the k-value of the low-k dielectric film 2 is due to the formation of silanol (Si—OH). In addition, the plasma damage can convert the surface of the normally hydrophobic low-k dielectric films, such as OSG, into hydrophilic films. Therefore, along with a degraded k-value, the surface of the low-k dielectric film can become vulnerable to chemical attack during exposure to wet chemical clean, which can result in significant loss of critical dimension (CD) control for low-k dielectric film comprising structures.
Thermal annealing of the low-k dielectric films, such as OSG, can negate some of the damage from plasma processing. Such thermal annealing removes some of the physically adsorbed, but unreacted moisture present at the surface of the low-k dielectric film. However, such annealing processes require annealing temperatures in excess of 250° C. for prolonged periods of time, such as on the order of 103 to 104 seconds, or annealing temperatures in excess of 400° C. for brief durations, such as on the order of 102 to 103 seconds. These temperatures and processing times are technically unfavorable, due to concerns of activating other thermal processes, such as copper stress migration. Also, significant equipment costs and increased cycle time are involved in such annealing processes. In addition, plasma-damaged low-k dielectric films that are annealed according to conventional processes, while removing the physically adsorbed moisture, can remain vulnerable to the re-adsorption of moisture and can result in the further formation of Si—OH in the low-k dielectric films.
Additionally, other properties of some low-k dielectric materials, such as in ULK dielectric materials, can lead to significant process integration challenges. ULK materials achieve lower k-values through the incorporation of non-polar covalent bonds (typically from the addition of carbon) and the introduction of porosity to decrease film density. These changes break the continuity of the Si—O—Si lattice of traditional oxides, leaving porous ULK dielectric films mechanically weaker than other dielectric films. Consequently, ULK dielectric films are more susceptible to kinetic plasma damage that can substantially increase their effective k-value. In some cases, the porous ULK films can collapse and thus densify, resulting in a higher k-value. Porous ULK films are also susceptible to the intercalation of plasma species, residues, solvents, moisture, and precursor molecules that can either adsorb into, outgas from, or chemically modify the film.